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Posting the error message you are seeing would help. Did you install/deinstall or change something else before your reboot?
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FuzzyQJan 14 '13 at 12:07

no i didn't..i dono how it happened also..after i installed and started using i didn't have any problem even when i reinstalled but it started after i shut down the laptop and started it the problem occured.
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user123169Jan 15 '13 at 15:59